"Her mom Tish went, and so did her boyfriend Liam [Hemsworth]," the source says of Cyrus. "The group was up until 3 a.m. just dancing up a storm and having fun."

Cyrus seemed quite impressed with Osbourne's planning skills, calling it "the best party ever" on Twitter. "Let's just say there was a unicorn in the lobby!" (An onlooker tells E! News there was a horse with a horn attached to its head inside the soiree.)